The present invention relates to a plug cock for controlling the supply of fuel in a small or a portable engine, wherein opening and closing of an air vent of a fuel tank is synchronized with that of a fuel supply passage.
The fuel cock of a compact engine is generally provided in the fuel supply passage communicating a fuel tank with a carburetor. On the other hand, the fuel tank must be opened to the atmosphere so as to avoid such problems as leakage of fuel caused by expansion of gases in the inner space of the fuel tank during the transportation of the engine or at high ambient temperature, and a shortage of fuel supply to the engine due to low pressure in the tank. Accordingly, an air venting device is provided. In such an air venting device, when the engine is off, or during the transportation of the engine by a vehicle, the fuel is vaporized thereby raising the pressure in the tank, so that the vaporized fuel leaks out of the tank causing odors. Therefore, the air passage of the air venting device is adapted to be able to open and close, for example, by providing a plug cock which is synchronized with a fuel plug. Japanese Utility Model Laid-open No. 55-176476 discloses a conventional plug cock having the above described structure.
In a such a plug cock, the plug had to be tightly engaged with a valve body and sealed with an O-ring in order to make it pressure-tight when the pressure in the fuel tank is high. However, if the O-ring wears out during use, the air passage of the air vent and the fuel supply passage communicate with each other, causing the vapor lock phenomenon wherein air is admitted into the fuel supply passage and the fuel supply to the carburetor is cut off. Therefore, the plug cock has to be disassembled to replace the O-ring periodically.
In addition, because of tight engagement of the plug, large torque is needed to rotate the plug. In addition, the torque is constant regardless of the level of pressure in the tank. Accordingly, even if the pressure is low, a large torque is necessary to rotate the plug.